Thermal switches are used in a variety of applications where it is desirable to activate and/or deactivate equipment as a function of sensed temperature. Such applications may include: rocket motors and thrusters, battery charge rate control, temperature control for fuel systems, environmental controls, overheat protection as well as many others. In several thermal switch applications, it is desirable to know when the switch has been activated and at what temperature. For example, it is desirable to know that the switch is functioning correctly when the switch is part of a safety system or is part of a control system used to protect equipment. Snap-action thermal switches are utilized in a number of applications, such as temperature control and overheat detection of mechanical devices such as motors and bearings. In some applications, multiple thermal switches are located at different positions around the equipment. For example, in some aircraft wing, fuselage, and cowling overheat detection applications, multiple thermal switches are located just behind the leading edge flap, while other thermal switches are spaced along the length of each wing. Additional thermal switches are located in the engine pylon and where the wing attaches to the fuselage. In this example, the multiple thermal switches are connected electrically in parallel, such that just two wires are used to interface between all of the switches on each wing and an instrument that monitors the temperature of the aircraft's wing, fuselage, and cowling.
Current snap-action thermal switch designs typically provide open and closed functions only. Typically, all of the thermal switches in the aircraft wing, fuselage, and cowling overheat detection applications are operated in the normally open state. The thermal switches are thus all in the “open” state until an overheat condition is detected, at which time one or more of the switches change to the “closed” state, thereby completing the circuit causing a “right wing,” “left wing” or “fuselage” overheat indication to appear in the cockpit. The pilot then follows the appropriate procedure to reduce the overheat condition.
Current snap-action thermal switches used in parallel operation, multiple thermal switch overheat detection systems suffer from various drawbacks. The integrity of the wire harness between the cockpit and the wing tip cannot be assured because the circuit is always open under normal operating conditions. If a switch connector is not engaged or the wire harness contains a broken lead wire, a malfunction indication will not occur, but neither will the overheat detection system operate during an actual in-flight overheat condition. Furthermore, if an overheat condition does occur, current snap-action thermal switches are not equipped to provide information describing the exact location of the overheat. In both instances, flight safety is compromised, and later correction of the problem that caused the overheat condition is made more difficult because of the inability to pinpoint the overheat fault.
One application for thermal switches that clearly illustrates the disadvantages of prior art devices is duct leak overheat detection systems. The duct leak overheat detection system is part of the aircraft deicing system. In this type of deicing system, hot air is forced pneumatically through a tube along the leading edge of the wing. Thermal switches located along this duct, indicate overheating, which could otherwise lead to structure failure and other system failures. When a thermal switch is tripped, a light illuminates in the cockpit indicating a “right” or “left” wing overheat condition. If, after shutting the system down on the appropriate wing, the switch does not reset, the airplane must divert to an emergency landing. Upon landing, the airplane maintenance personnel have no way of knowing which particular switch has been activated, because there exist multiple thermal switches linked to a particular cockpit light. The existing airplane systems have only provided the crew with an indication of the particular wing semispan along which a thermal switch was tripped. If the switch has reset, there is no indication to the maintenance personnel that it was tripped by the overheat condition. This dearth of information requires the crew to physically access and inspect the entire system along the appropriate wing semispan. Even in applications where only one temperature probe indicated an alarm temperature in-flight, extensive and expensive troubleshooting is sometimes necessary. For example, an airborne alert from a temperature probe in aircraft turbine bleed air ductwork may require engine run-up and monitoring on the ground to determine whether the probe and/or the bleed air system is faulty.